Gas springs are utilized typically to yieldingly resist movement of a body such as a clamping ring for a metal blank in a die in a press for forming sheet metal parts.
Gas springs are generally constructed with an actuating rod connected to a piston or head slidably received in a cylinder having a chamber which is charged at a predetermined pressure, such as 2000 PSI, with an inert gas, such as nitrogen. This provides a gas spring or cushion permitting the rod to yieldably move toward its retracted position when a force applied externally to the rod exceeds the force produced thereon by the gas in the chamber. When the rod and head are forced into the chamber, the gas therein is compressed to a maximum operating pressure which is usually in the range of about 3,000 to 5,000 PSI, depending on the volume of the chamber and the effective area and stroke of the rod and piston. In normal use, the pressure to which a self-contained gas spring is initially charged is not varied or changed. The spring is initially charged, relieved and recharged through a high pressure valve of the type commonly used in aircraft struts and accumulators.
Previously, gas springs have been constructed with the rod being slidably received in a bearing and seal housing assembly received in the cylinder. This housing assembly has a rear housing slidably received on a front housing with a rod gasket or seal therebetween. In use, the seal is compressed axially so that it expands radially inward into firm sealing engagement with the rod by the force produced by pressurized gas in the chamber acting on the rear housing. The movement of the housing causes wear on adjacent parts and wear of the housing itself. Such a gas spring is shown in U.S. Pat. No. 4,792,128, which has a positive stop preventing the head from striking the housing assembly and damaging the bearings and seals.
U.S. Pat. No. 4,838,527 shows a gas spring which can be operated with either an external source of gas or as a precharged and self-contained unit by changing first and second valves assemblies either of which can be installed in the same cavity in the cylinder.
These gas springs require a substantial force to begin moving their rods toward their retracted positions. This force increases rapidly as their rods are retracted. The force required to begin this retraction stroke is often so substantial that a mechanical crankshaft type press may have difficulty producing sufficient force to retract the rod. This lengthens the press cycle time or even stalls the press. This may occur because the press produces relatively little force at the beginning of its stroke and most of its force near the very end of its stroke. With conventional presses and dies relatively little force need be applied to the clamping ring during the initial portion of the press stroke and a much greater force is required near the end of the press stroke. Excessive forces cause excessive wear on the crank, crank bearings and die parts. Thus, the desired force curve for press operations is opposite to that produced by prior gas springs.